Raymond (88persuader) to change Impedance on a 4x10 8 Ohm you must change each Speaker on it. There is some devices that do this eletronically, but I can't remember the Label or product name.
As far as I can figure it out, your Cab may have four 8 Ohm Speakers wired in series/parallel. Probably two 8 Ohm Speakers wired in series, summing 16 Ohm each pair, and both pairs wired in parallel finally summing 8 Ohm on Cab.
So if you want to change your Cab to 4 Ohms you should use only 4 Ohm Speakers wired in same fashion... Not a cheap convertion.
(Message edited by Mario Farufyno on September 17, 2010)